[3.1-RC1] Add a non-normative 'known consumers' appendix

[henrylyons]

Affected Sections: ecosystem

Suggested Change:
Add a non-normative 'known consumers' appendix listing major SPDX-consuming pipelines and their version-support roadmaps so implementers can find downstream context. (ecosystem)

Notes:
This feedback was provided by Microsoft at the SPDX working group meeting on June 30, 2026.

[bact]

Annex - should move to spdx-spec repo.